1. Field of the Invention
This invention relates to a digital signal recording apparatus and more particularly to a digital signal recording apparatus which is capable of changing the amount of information to be recorded per unit time from one value over to another.
2. Description of the Related Art
To make alterable the length of recording time on one and the same recording medium, conventional apparatuses for recording an analog signal of a wide frequency band, such as a video signal, have generally adopted a method of changing a recording track pitch, as in the case of a video tape recorder (hereinafter referred to as VTR). This method increases the recordable length of time at the expense of the dynamic range of the S/N ratio of a recording signal. The VTR of this kind is thus arranged to permit track pitch setting as desired without much difficulty and can be set at a desired length of recording time with comparative ease. Therefore, there has been no technical difficulty in arranging the VTR to permit recording time setting at various values.
Meanwhile, the advancement of digital signal processing technology of recent years has resulted in the development of a digital video tape recorder (hereinafter referred to as D-VTR) which is arranged to digitize the analog signal of a wide frequency band, such as a video signal, and to record or reproduce the signal by performing digital modulation before recording. The S/N ratio of the signal reproduced by the D-VTR is independent of the quality of the reproduced information. However, when the S/N ratio is below a certain value, reproduction itself becomes impossible. In the case of the D-VTR of this kind, therefore, the arrangement to make the recording time variable by changing the track pitch is meaningless as the image is recorded or reproduced at the same quality at any track pitch selected. In the case of the D-VTR, therefore, signal recording or reproduction is preferably performed at a track pitch which is predetermined to allow adequate reproduction of digital data. In this case, it is hardly conceivable to make any arrangement for varying the recording time by changing this track pitch.
In view of this, for the D-VTR of this kind, long-time recording has been made possible by reducing the amount of data to be recorded. In other words, the recording time is arranged to be variable by changing the number of tracks to be formed per unit time.
A method for changing the recording time in this manner has been disclosed in Japanese Laid-Open Patent Application No. SHO 61-77104. This method is as follows: FIG. 1 of the accompanying drawings shows the head arrangement of the conventional VTR of the above stated kind. A magnetic tape T is caused by tape guide posts 101a and 101b to be wrapped at least 180 degrees around a rotary cylinder 102. Four rotary heads Ha-, Ha+, Hb- and Hb+ are mounted on the cylinder 102 and are spaced at a phase difference of 90 degrees from each other. These heads are disposed at the same height in the direction of the axis of rotation. The heads Ha+ and Hb+ have a first azimuth angle (hereinafter referred to as a plus azimuth) and the heads Ha- and Hb- a second azimuth angle (hereinafter referred to as minus azimuth).
In a standard recording mode, a recording signal is supplied to all the heads tracing the surface of the tape T. The heads record the signal while forming tracks one after another in rotation in the order of head Ha+, head Ha-, head Hb+ and head Hb-. FIG. 2(A) shows a recording pattern obtained in the standard recording mode of the VTR having the head arrangement as shown in FIG. 1. The heads Ha+, Ha-, Hb+ and Hb- respectively form tracks Ta+, Ta-, Tb+ and Tb-.
In a long-time recording mode, the tape T is transported at a speed which is 1/3 of the speed of the standard recording mode. Meanwhile, the rotary cylinder 102 is rotated in the same direction and at the same speed as in the standard recording mode. In this case, the amount of recording data is compressed to 1/3. The head Hb- performs recording after completion of recording by the head Ha+. More specifically, the heads traces the surface of the magnetic tape T in rotation in the order of head Ha+, head Ha-, head Hb+ and head Hb- every time the cylinder 102 turns around 90 degrees in the same manner as in the case of the standard recording mode. However, the heads Ha- and Hb+ do not perform signal recording after completion of recording by the head Ha+. Accordingly, the recordable amount of data per unit time is reduced to 1/3 thereof. However, the above stated data compression makes it possible to record the same time length portion of the video signal.
FIG. 2(B) shows a recording pattern obtained in the long-time recording mode of the VTR shown in FIG. 1. The same reference symbols as those used in FIG. 2(A) denote the same tracks. The long-time mode somewhat differs form the standard mode in the inclination of the tracks. However, the track pitches obtained in these two modes are almost equal to each other if the difference between the track angles .phi.a and .phi.b shown in these drawings is sufficiently small. In short, the same time length portion of the video signal is recordable in two tracks in the long-time recording mode as in six tracks formed in the standard recording mode.
There are varied methods for changing the recordable amount of data. In one method, for example, the number of bits for each picture element of the video signal is changed. In another method among others, sampling frequency is changed.
However, in performing the so-called azimuth recording with the D-VTR, the head of the plus azimuth and the head of the minus azimuth are used alternately with each other. Therefore, in relation to the mode in which all the tape tracing heads perform recording (the standard recording mode int he case of the prior art VTR described), data compressible rates are limited to values which are one odd fraction of the amount of data.
Meanwhile, the currently known data compressing methods include many methods in which data is arranged to be compressed to 1/2 by sampling or frame reduction or the like. In view of these methods, the VTR of the above stated kind arranged to permit setting it at one of a plurality of recording time values is preferably arranged to permit selection of any desired value for the long-time recording mode from among the data compressing rates relative to the standard recording mode.
Further, if the D-VTR of the kind described is to be arranged to have the recordable amount of data per unit time changeable to any of varied values, varied data compression rates are necessary. Then, to have an n-number of different selectable recording time values, for example, the VTR must have (n-1) channels of a digital signal processing system. This requirement inevitably results in a complex arrangement of the VTR.